1. Field of the Invention
The present invention relates to a secondary cell, particularly to a lithium polymer secondary cell.
2. Description of the Related Art
Recently, lithium secondary cells are drawing attention among other secondary cells, since they have high energy densities and can realize lightening of the cells.
The lithium secondary cell has, for example, a structure which comprises; a positive electrode having a positive electrode layer such as of lithium cobaltate (LiCoO.sub.2) formed on a positive electrode collector; and a negative electrode having a negative electrode layer such as of graphite formed on a negative electrode collector, both the electrodes are disposed in a sealed vessel filled with an organic electrolyte solution to be spaced from each other by a separator.
Meanwhile, there are developed lithium secondary cells employing polymeric positive electrode layer materials such as polyanilines (hereinafter abbreviated sometimes as PAn), in place of LiCoO.sub.2 used as the positive electrode layer material, for the purpose of weight reduction and for safety's sake. And also there are developed lithium secondary cells containing additionally in the positive electrode layer a thiol compound having a great theoretical capacity such as 2,5-dimercapto-1,3,4-thiadiazole (hereinafter abbreviated as DMcT), which increases storable energy density.
In positive electrode layer materials, thiol compounds each having a mercapto group (--SH) in the molecule such as DMcT undergo reversible formation and dissociation of the disulfide bonds (--SS--) as redox reactions take place, so that those compounds which have two --SH groups in the molecule like DMcT each liberate two electrons per molecule in the oxidation reaction to undergo a polymerization reaction and form a polydisulfide bond, whereas they each undergo a depolymerization in the reduction reaction to return to the DMcT monomer. Further, DMcT has insulating properties both in the form of oxidant and in the form of reductant. Accordingly, it is necessary to combine DMcT with a conductive material so that it can be used as an electrode material. Such conductive materials include carbon powders or fibers such as graphite powders, graphite fibers and acetylene black (hereinafter abbreviated sometimes as AB) powders, and conductive polymers such as polypyrroles excluding PAn.